This invention relates to a magnetic powder and a permanent magnet having magnetic properties enhanced by taking advantage of a magnetic interaction and a process for producing them.
In general, permanent magnetic materials have a tendency that an enhancement in saturation magnetization (or residual magnetic flux density) is not compatible with a high coercive force. More specifically, the following tendency is observed.
Soft magnetic materials are those materials which have a high saturation magnetization. For example, permendur has such a high saturation magnetization of 24 kG. It, however, has little or no coercive force.
On the other hand, hard magnetic materials with a high coercive force, however, have much lower saturation magnetization than that of the soft magnetic materials. Among the hard magnetic materials, R.sub.2 Fe.sub.14 B-based, R.sub.2 Fe.sub.17 N.sub.x -based and R.sub.2 TM.sub.17 -based materials have a relatively high saturation magnetization.
In the R.sub.2 Fe.sub.14 B-based materials, in order to enhance the saturation magnetization, it is necessary to reduce the volume fraction grain boundary phase and maximize the volume fraction of the R.sub.2 Fe.sub.14 B phase as a main phase. A volume reduction in the grain boundary phase, however, makes it difficult to separate each grain of main phase, resulting in a low coercive force. When R is Nd, a high saturation magnetization is obtained. On the other hand, in order to obtain a high coercive force, it is a common practice to substitute Dy or the other heavy rare earth element for part of Nd. The substitution with Dy lowers the saturation magnetization.
The saturation magnetization of the R.sub.2 Fe.sub.17 N.sub.x -based material (particularly when R=Sm) is nearly equal to that of Nd.sub.2 Fe.sub.14 B. However, in order to obtain a coercive force, the powder particle diameter must be pulverized to several .mu.m, so that the coercive force obtained is substantially small for practical use. Further, since the material has to be a finely milled, when it is compacted into a bonded magnet or the like, the packing density of magnetic powder can't be raised. The addition of V, Mn or the like makes it possible to obtain a high coercive force in a relatively large powder particle diameter. It, however, results in a lowered saturation magnetization.
R.sub.2 TM.sub.17 -based (particularly R=Sm) bonded magnets are reported in many documents such as Japanese Patent Publication Nos. 22696/1989, 25819/1989 and 40483/1989 and patents and papers cited therein. Especially, an attempt to increase the Fe content of TM has been made as a means for improving the performance of this system. In this attempt, as described in FIG. 2 of Proc. 10th Int. Workshop on Rare Earth Magnets and Their Applications, 265 (1989), the maximum energy product (BH).sub.max shows a peak value when the Fe content is a certain value. As suggested in Proc. of 11th Rare Earth Research Cont., 476 (1974), this is attributable to the fact that an increase in Fe content contributes to an increase in saturation magnetization but unfavorably lowers the magnetic anisotropy. For Sm.sub.2 Co.sub.17 -based bonded magnets having a high Fe content, as described in Proc. of ICF6, (1992) p1050-1051, fine cast structure and optimum heat treatments prevent a lowering in coercive force and squareness (due to the increase in Fe content), so that increase the performance. Further, as reported in Japanese Patent Laid-Open No. 218445/1985 and papers, in some cases, an improvement in performance is attempted by employing, as Rare Earth element, Sm part of which has been substituted with other Rare Earth elements rather than use of Sm alone. As described in FIG. 1 of IEEE Trans. Mag. MAG-20, 1593 (1984), Table 1 of IEEE Trans. Mag. MAG-15, 1762 (1979) and some documents, among R's, a Pr or Nd substituted system can increase the saturation magnetization in accordance with an increase in substituted volume, but results in a lowering in magnetic anisotropy. Bonded magnets comprising the above composition system are described in Journal of The Magnetics Society of Japan, 11, 243 (1987), Journal of the Japan Society of Powder and Powder Metallurgy, 35, 584 (1988) and the like.
Bonded magnets produced by mixing two rare earth magnetic powders together are disclosed in Japanese Patent Laid-Open Nos. 144621/1993 and 152116/1993 and the like. The bonded magnet disclosed in Japanese Patent Laid-Open No. 144621/1993 (Applicant: Tokin Corp.) comprises a mixture of an R.sub.2 Fe.sub.17 N-based powder with an R.sub.2 Co.sub.17 -based powder, and the bonded magnet disclosed in Japanese Patent Laid-Open No. 152116/1993 comprises a mixture of an R.sub.2 Fe.sub.17 N-based powder with an R.sub.2 Fe.sub.14 B-based powder. However, neither information on coercive force of the mixed powder nor an improvement in magnetic properties by magnetic interaction among powder particles is disclosed, and the improvement in magnetic properties by mixing relies entirely upon an enhancement in packing density of magnetic powder (see Japanese Patent Laid-Open No. 144621/1993 on page 2, right col., line 24 and Japanese Patent Laid-Open No. 152116/1993 on page 2, right col., line 34 to page 3, left col., line 9). Furthermore, Japanese Patent Laid-Open No. 36613/1992 discloses that powders different from each other in particle diameter and coercive force are mixed together. But in this proposal, the coercive force and the particle diameter are not limited at all, and nothing is mentioned on an improvement in squareness by the magnetic interaction.
In recent years, the magnetic materials called an "exchange spring magnets" have been reported in the art. These magnets comprise a soft magnetic phase and a hard magnetic phase. The thickness of the soft magnetic phase is made smaller than the domain wall width of the soft magnetic phase to inhibit the magnetization reversal of the soft magnetic phase, thereby enabling coercive force to be increased. More specifically, .alpha.Fe--Nd.sub.2 Fe.sub.14 B, Fe.sub.3 B--Nd.sub.2 Fe.sub.14 B, .alpha.Fe--Sm.sub.2 Fe.sub.17 N.sub.x and other materials have been reported. In the above exchange spring magnets, the phases must be crystallographically coherent. Among processes for producing the above materials include rapid quenching and mechanical alloying. These production processes impose restriction on a combination of the soft magnetic phase with the hard magnetic phase. Further, the structure renders the squareness low. Furthermore, at the present time, these magnetic materials which could have successfully produced in the art are isotropic, and anisotropic magnetic materials have not been reported at all.
Accordingly, the conventional permanent magnets had the following problems.
(1) An increase in saturation magnetization gives rise to a decrease in coercive force, which results in a decrease in maximum energy product (BH).sub.max.
(2) An increase in coercive force unfavorably gives rise to a decrease in saturation magnetization.
(3) In mixing of two powders having different properties, an improvement in magnetic property appears only in the form of the sum of each properties of the two powders, and no improvement in the properties beyond the sum can be obtained.
(4) The magnetic powder comprising two phases (exchange spring magnet) cannot provide anisotropic characteristics.